1. Field of the Invention
The present invention relates to a method of performing calibration and quality control of a sensor for determining a parameter in a test fluid as well as an apparatus for performing the method.
2. Discussion of the Related Art
Sensors for measuring a parameter in a test fluid are widely used in various fields of industries. Examples of industries are the food industry, the environmental industry and the medical and the clinical industry, in particular the clinical laboratory industry.
In order to assure that sensor measurements are accurate it is required to regularly calibrate the sensors, i.e. to determine experimentally the correspondence between sensor responses and predetermined parameter values of the reference materials, usually by measurements on one or more reference materials.
In many cases it is also required to regularly control the quality of the sensor performance, i.e. to verify experimentally that the sensor measurements are accurate and/or precise, usually by comparison of a measured parameter value of a reference material with an acceptance range of the same reference material.
In the food industry such routines of calibration and quality control are performed for instance on sensors for assessing milk quality. In the medical and clinical industry such calibration and quality control routines are performed for instance on sensors for determining parameters in physiological fluids.
DE 41 04 302 discloses a method of performing quality control and optionally calibrating a sensor for determining a parameter in a physiological fluid. According to the method, the same volume of reference solution may be used to control and, if needed, to calibrate a measuring cell. The advantage is that the same reference solution may be used both in measuring cells for determining the hematocrit value and in cells for determining physiological electrolytes. In the method, however, the same volume of reference solution is used for both calibration and quality control of the sensor, and therefore the method does not provide for an independent assessment of sensor performance.
The general requirement for reference materials for performing calibration and quality control routines is, that the reference material used in quality control of a sensor must be different from the reference material(s) used for the calibration upon which the quality control is based in order to ensure that the quality control procedure provides an independent assessment of sensor performance.
In the clinical and medical industry, legislation requires that sensors are exposed to quality control procedures in addition to regular calibration procedures. The quality control procedures must be performed often enough and covering a broad enough parameter range to be able to prove, at any time, that the sensor used is able to provide reliable data in its entire measuring range. This means that sensors must be checked by means of a system, which is independent of the normal calibration system of the sensor.
It is accordingly recommended that in successive quality control cycles the quality of a sensor is controlled using a number of reference materials representing different parameter levels (often low, middle and high).
The same reference material may represent several different parameters at a time. This way calibration and/or quality control may be performed on several sensors simultaneously using only one reference material. One level of one parameter may be combined with any level of other parameters. An example of such a reference material is found in U.S. Pat. No. 5,910,445 disclosing quality control liquids for use in the quality control procedure of electrochemical measuring apparatuses for measurement on physiological liquids.
Further, U.S. Pat. No. 4,701,417 discloses calibration and control sera for use in lipid diagnosis. Solutions for calibrating and/or quality controlling sensors in blood gas analysers are disclosed in, for example, U.S. Pat. Nos. 5,422,278 and 5,910,445. In all cases the reference materials for calibration and the materials for quality control are described as two separate groups of reference materials.
In automatic apparatuses for blood gas analysis it is standard procedure to use at least five different reference materials in the calibration and quality control of the respective sensors. An example is the blood analyser ABL™725 manufactured by Radiometer Medical A/S. In this apparatus several different blood parameters are determined. It contains two calibration solution containers which are integrated in the apparatus and which need to be renewed approximately once a month.
In the ABL™725 blood analyser, the quality control may be performed manually by a specially trained operator at specific hours by aspirating reference solution from an ampoule. After shaking and breaking open the ampoule, it is provided with an adapter for attaching it to the inlet upon which an automated quality control procedure is initiated. When the operator has obtained a measuring result, he or she must determine whether the result falls within a certain acceptance range, which is given on an insert of the ampoule. It is recommended that reference materials of at least three different parameter levels are used.
Alternatively, the ABL™725 blood analyser may be provided with a module (AutoCheck™) for automatic aspiration of control fluid from an ampoule for quality control of the apparatus. With this module, operator handling is reduced to removing a holder from the apparatus, loading the holder with ampoules and reinserting it. This must be done approximately once a week. The operator still needs to pay attention to the number of ampoules of each level that he or she loads into the holder.
Thus, in the known methods of performing calibration and quality control of a sensor for determining a parameter in a test fluid, a substantive number of different reference materials are required. Also, the known methods require that a specially trained operator perform the quality control routine or at least once a week load a holder with ampoules for automatic aspiration of quality control fluid.